1. Field of the Invention
The present invention relates to a method for evaluating the quality of a lens to be used in an optical apparatus such as a projector, which includes the steps of: illuminating imaging light including a test pattern for the measurement of resolution on a screen through the lens; and detecting the brightness of the test pattern image displayed on the screen by an image-capturing device having an image sensor, and calculating the evaluated value of resolution on the basis of the detected brightness level. Also, the present invention relates to a lens-evaluating apparatus using such a lens-evaluating method.
2. Description of the Related Art
Heretofore, there has been used a projector that comprises: a plurality of liquid crystal panels for modulating a plurality of color light beams for each of them on the basis of image information; a cross-dichroic prism for combining color light beams modulated by each of the liquid crystal panels; and a projector lens for forming a projecting image by extending and projecting a beam of light on a screen to form a projected image.
The projector lens used in such a conventional projector may cause variations in its optical characteristics such as image resolution, flare, chromatic aberration, curvature aberration, and the distribution of illuminance of the projected image due to variations in its manufacturing process or the like. Variations in the characteristics of the projector lens influence on the quality of an image displayed on a screen by the projector. Therefore, the characteristics of the lens are evaluated before the shipment of the lens and before the assembly of a projector with the lens.
Concretely, for example, the resolution of the projector lens is evaluated by forming a test pattern for the resolution measurement on a test sheet, illuminating light on the test pattern to generate imaging light that includes a test pattern image, introducing the imaging light into a projector lens provided as a testing sample to be evaluated, and projecting the imaging light from the projector lens on the screen. Then, the test pattern image displayed on the screen is detected by an image capturing apparatus using an image sensor such as a charge coupled device (CCD), followed by subjecting the detected image using a computer or the like to evaluate the resolution of the projector lens.
Similarly, when the projector lens is evaluated for the generation of flare, it can be performed using a flare-testing pattern formed on a test sheet. When the projector lens is evaluated for the generation of chromatic aberration, a color beam filters can be used. Each of the color light beam filters has a function of extracting a color light beam which corresponds to a specific color included in the light emitted from a light source. In other words, the light beam passed through the color beam filters of red, green, and blue illuminates a screen to represent an image of test pattern. Then, the image displayed on the screen is detected by an image-capturing device using an image sensor such as CCD. Subsequently, the detected image is subjected to an arithmetic operation by a computer or the like to evaluate the chromatic aberration of the projector lens.
Furthermore, for evaluating the curvature aberration of the projector lens and the illuminating distribution of a projected image, an image of test pattern displayed on the screen is visually observed to check the qualities.
Conventionally, a basic lens has been applied in the method for evaluating the quality of a lens. In such a method, the deviation between the basic lens and a target lens to be evaluated is measured to evaluate the quality of the target lens. Specifically, the basic lens that shows an average level of each characteristic of the lens is used for the evaluation, where a test sheet is adjusted to be positioned on the back-focal surface of the basic lens. Then, an imaging beam illuminates a screen through the basic lens. Subsequently, an image sensor positioned on each of four corners of the screen scans a test pattern formed on the test sheet by means of a pattern matching. The test pattern is imaged and is then provided image data. From the resulting image data, it is judged whether focus is achieved using a specific indexical value (edge strength) of the test pattern image to check the quality of focusing. As a result, four indexical values can be obtained. Depending on the resulting four indexical values, a six-axis adjusting part is controlled to adjust the spatial arrangement of the test sheet (test pattern) such that these four indexical values become almost equal to each other and become the maximum values. Subsequently, in a state of fixing the test sheet being justified using the basic lens, the target lens to be evaluated is subjected to the characteristic evaluation of the lens.
Therefore, the evaluation of optical characteristics of the lens in quick motion can be performed because of no need to adjust the focus of the target lens on the test sheet.
However, the back-focal surface of the actual lens is curved. Such a curvature of the back focus of the lens is unique to an individual lens. In other words, the positioning of the test sheet is performed using the basic lens, and the evaluation of the quality of each lens is performed on the position of the test sheet being adjusted by the basic lens. For the evaluation of optical characteristics of each lens, there is a problem that the quality of the lens can be evaluated under the conditions in which the test sheet (test pattern) and the back-focal surface of each lens are misaligned with respect to each other.
If the evaluations are carried out to check the curvature aberration of the projector lens and the illuminating distribution of the projected image by visually observing the projected image, the exact characteristic value of the project lens cannot be obtained and a criterion of judgment whether the characteristic value is acceptable or not is obscure.
A primary object of the present invention is to provide a method for properly evaluating the characteristics of a lens and a lens-evaluating apparatus using such a method.
A method for evaluating the quality of a lens (also referred to as a lens-evaluating method) in accordance with present invention comprising: illuminating imaging light on a screen through the lens to form a projected image, where the imaging light having a test-pattern image is generated using a test sheet on which a test pattern for measuring a resolution of the lens is formed to evaluate the resolution; detecting a brightness of the test-pattern image displayed on the screen by an image-capturing device using an imaging sensor; calculating an input level on the basis of the detected brightness of the test-pattern image; and calculating an evaluated value of resolution, and adjusting a position of the test sheet to a position corresponding to a focus of the lens by detecting the test-pattern image while moving the test sheet back and forth in the direction along an optical axis of the lens.
Here, the imaging element may be selected from a CCD (charge-coupled device) sensor, a MOS (metal oxide semiconductor) sensor, and so on. Also, the image-capturing device may be a device for obtaining image data, such as a video capture board that receives an output from the imaging element and converts it into an image signal to be transmitted to a computer.
According to the present invention, as the method has the step of adjusting the focus of the projector lens, the evaluated value of resolution can be calculated on the basis of imaging light detected under the condition in which the position of the test sheet is adjusted to the focal position of the test lens. Therefore, it becomes possible to correctly calculate the evaluated value of resolution by preventing the image from out of focus or the like due to a bend in the back-focal surface of the lens or the like.
In addition, for focal adjustment of each lens, the deviation obtained by shifting the test sheet in the direction along the optical axis may be recorded to estimate the variation in the process of manufacturing individual lens, allowing the operator to grasp the optical characteristics of the lens with a high degree of accuracy.
In the above description, preferably, the method for evaluating the quality of a lens may comprise: switching the imaging light including the test-pattern image into a plurality of color light beams; and measuring a chromatic aberration of the lens on an optical axis thereof by detecting a plurality of imaging light beams corresponding to the plurality of color light beams being switched.
Here, the chromatic aberration of the lens can be measured by acquiring the focal position by moving the test sheet back and forth in the direction along the optical axis of the lens on the basis of a switched color light beam.
According to the present invention, as the method has the step of switching color light beams and the step of measuring chromatic aberration, the optical characteristics of the lens can be measured with a high degree of accuracy because of the measurement of chromatic aberration in addition to evaluate the resolution of the test lens.
Furthermore, there is no need to form an additional pattern for the measurement of chromatic aberration on the test sheet, so that the test pattern on the test sheet can be simplified.
Preferably, the lens may be constructed as a combination of lenses in which a plurality of converging elements is arranged in the direction along the optical axis of the lens. If a zooming mechanism is provided for allowing the projected image to be scaled up or down by changing a relative position of each of the converging elements, the steps of illuminating the imaging light, detecting the imaging light, calculating the input level, and calculating the evaluated value may be performed at least at a minimum magnitude of the lens and a maximum magnitude of the lens.
As described above, the evaluation of resolution can be performed at the minimum and maximum magnifications of the lens. Therefore, the resolution of a scalable combination of lenses can be evaluated even if it is scaled up or scaled down with a high degree of accuracy.
Preferably, furthermore, if the imaging element is constructed so as to be movable along the surface of the screen, the method may further comprise the steps of: moving the imaging clement along an outer peripheral end of the project image on the screen; acquiring an end image by the image-capturing device using the imaging element at a predetermined position during the movement of the imaging element by the step of moving the imaging element; and calculating the amount of curvature aberration of the projected image on the basis of the end image of the projected image acquired by the step of acquiring the end image.
According to the present invention, the imaging element is capable of moving along the surface of the screen. As the methods includes the steps of moving the image element, acquiring the end image, and calculating the amount of curvature aberration, the imaging element is allowed to move along an outer peripheral end of the project image on the screen. Thus, an end image can be acquired by the image-capturing device using the imaging element at a predetermined position during the movement of the imaging element by the step of moving the imaging element. Therefore, it eliminates ambiguity over the accuracy of evaluation, which has been performed by visual observation, resulting in the correct evaluation of curvature aberration.
Preferably, furthermore, if the test sheet includes a frame portion formed in the vicinity of a periphery of an area on which the projecting image is formed, the step of acquiring the end image may acquire an image of the frame portion formed on the screen.
Furthermore, as the image of the frame-shaped shading portion is acquired by the step for the acquisition of the end image, the brightness levels of the image of the shading portion can be obtained within the range between the bright area on the outside of the frame-shaped portion and the bright area on the inside of the shading portion, the predetermined brightness level can be used as a threshold to grasp positions corresponding to such a threshold, by which the frame-shaped portion is sandwiched. Therefore, the middle point is calculated from these positions, so that the coordinates of the middle of the image of the shading portion can be easily grasped. Thus, such coordinates can be used to acquire the amount of aberration with a high precision in contrast to the designed value and to evaluate the curvature aberration of the lens can be evaluated with a higher degree of accuracy.
Preferably, in the above method for evaluating the quality of a lens, if the steps of illuminating the imaging light, detecting the imaging light, calculating the input level, and calculating the evaluated value are performed on a plurality of positions within the projected image, the method may further comprise the steps of: acquiring an illuminance at a predetermined position, on which the steps of illuminating the imaging light, detecting the imaging light, calculating the input level, and calculating the evaluated value are performed, in the projected image; and calculating an in-plane illuminance of the whole projecting image by calculating an illuminance of other position on the basis of the input level and the illuminance of the predetermined position and an input level of other position.
Here, the specified calculation of in-plane illuminance can be performed with the following equation.
Le=Loxc3x97Iie/Iio
wherein lie denotes an input level of the other position, lio denotes an input level of the predetermined position, and Lo denotes an illuminance of the predetermined position.
According to the present invention, the illuminance at other position can be calculated on the basis of input levels of a plurality of positions only by acquiring the illuminance at a predetermined position through the illuminometer. Therefore, the in-plane illuminance of the projected lens can be acquired by the measurement of illuminance only at one predetermined position in the projected image, grasping the optical characteristics of the lens with a higher accuracy.
An apparatus for evaluating the quality of a lens (also referred to as a lens-evaluating apparatus), comprises: a test sheet on which a test pattern for evaluating a resolution of a lens is formed; a light source for introducing imaging light into the lens by illuminating light on the test sheet, where the imaging light includes a test-pattern image generated by the test pattern; a screen on which the imaging light is projected from the lens; an image sensor for imaging the test-pattern displayed on the screen; an image-capturing part for generating an image signal by capturing an image taken by the image sensor; and a signal-processor including a calculator for obtaining an evaluated value of resolution, in which an input level and the evaluated value of resolution are computed on the basis of the image signal outputted from the image-capturing part, wherein the signal-processor has a means for adjusting a position of the test sheet to a position corresponding to a focus of the lens by detecting the test-pattern image while moving the test sheet back and forth in the direction along an optical axis of the lens.
According to the present invention, as the apparatus has the means for adjusting the focus of the projector lens, the evaluated value of resolution can be calculated on the basis of imaging light detected under the condition in which the position of the test sheet is adjusted to the focal position of the test lens. Therefore, it becomes possible to obtain the same actions and the same effects as those obtained by the method for evaluating the quality of the lens.
In the above description, preferably, the apparatus for evaluating the quality of a lens may further comprise: a switching part for switching the imaging light including the test-pattern image into a plurality of color light beams; and a measuring part for measuring a chromatic aberration of the lens on an optical axis thereof by detecting a plurality of imaging light beams corresponding to the plurality of color light beams being switched.
According to the present invention, as the apparatus has the switching part for switching color light beams and the measuring part for measuring chromatic aberration, it is possible to obtain the same actions and the same effects as those obtained by the method for evaluating the quality of the lens because of the measurement of chromatic aberration in addition to evaluate the resolution of the test lens.
Preferably, the apparatus for evaluating the quality of a lens in accordance with the present invention may further comprise: a mechanism for moving the imaging element along the surface of the screen, wherein the signal processor includes: an imaging element controller for controlling a movement of the imaging element along an outer peripheral end of the projected image on the screen; an end image detector for acquiring an end image by the image-capturing device using the imaging element at a predetermined position during the movement of the imaging element by the imaging element controller, and a calculator for calculating the amount of curvature aberration of the projected image on the basis of the end image of the projected image acquired by the end image detector.
According to the present invention, there is provided a mechanism for moving the imaging element along the surface of the screen, and the signal processor includes the imaging element controller, the end-image detector, and the calculator for calculating the amount of curvature aberration. Therefore, the amount of curvature aberration of the lens can be calculated by the calculator for calculating the amount of curvature aberration, so that it becomes possible to obtain the same actions and the same effects as those obtained by the method for evaluating the quality of the lens.
Preferably, in the apparatus for evaluating the quality of a lens in accordance with present invention, the test sheet may include a frame portion formed in the vicinity of a periphery of an area on which the projecting image on the screen is formed.
According to the present invention, the test sheet has frame portion formed in the vicinity of a periphery of an area on which the projecting image on the screen is formed, so that the imaging element can be moved along the periphery of the frame portion by the image element controller to acquire the end image of the frame portion at the predetermined position by the end image detector, allowing the calculation of the amount of curvature aberration based on the resulting end image by the calculator of the amount of curvature aberration. Therefore, the signal processor may easily obtain the end image of the frame portion to calculate the amount of curvature aberration, so that the amount of curvature aberration of the lens can be quickly evaluated with a high accuracy.
Preferably, in the apparatus for evaluating the quality of a lens in accordance with present invention, the test sheet may include a flame portion formed in the vicinity of a periphery of an area on which the projecting image on the screen is formed.
According to the present invention, the test sheet has a flame portion formed in the vicinity of a periphery of an area on which the projecting image on the screen is formed, so that the imaging element can be moved along the periphery of the flame portion by the image element controller to acquire the end image of the flame portion at the predetermined position by the end image detector, allowing the calculation of the amount of curvature aberration based on the resulting end image by the calculator of the amount of curvature aberration. Therefore, the signal processor may easily obtain the end image of the frame portion to calculate the amount of curvature aberration, so that the amount of curvature aberration of the lens can be quickly evaluated with a high accuracy.
Preferably, furthermore, the apparatus for evaluating the quality of a lens in accordance with the present invention may further comprise an illuminometer that detects an illuminance of a predetermined position in the projected image.
According to the present invention, as the apparatus includes the illuminometer that detects the illuminance of a predetermined position in the projected image, the difference in illuminance caused by the lens can be evaluated by making a comparison between the detected illuminance levels for each lens under examination.
Preferably, furthermore, in the apparatus for evaluating the quality of a lens in accordance with the present invention, if the acquisition of an input level by the calculator for obtaining the evaluated value of resolution is performed at each of a plurality of positions in the projected image including a predetermined position where the illuminance is detected, the signal processor may include a calculator for calculating an in-plane illuminance of the whole projecting image by calculating an illuminance of other position on the basis of the illuminance of the predetermined position detected by the illuminance detector, the input level of the predetermined position calculated by the calculator for calculating the evaluated value of resolution and the input level of the other position.
According to the present invention, therefore, as the signal processor includes the calculator for obtaining the in-plane illuminance, the in-plane illuminance can be calculated by such a calculator using the same procedure as that of the method of evaluating the quality of the lens. Therefore, it becomes possible to obtain the same actions and the same effects as those obtained by the method for evaluating the quality of the lens.